Indeed, it is particularly beneficial for a user of such articles to see the change in temperature of an article during use when the latter is subjected to heating. In the case of a cooking article, good control of temperature while cooking foods is necessary for sanitary and gustatory reasons (for example, for searing a steak on a grill or in a skillet), and for limiting occasional overheating that can weaken the coating of the cooking article. In the case of a hair iron, a thermochromic functional indicator enables the user to know the optimal temperature of use of the hair iron: in this particular case, it concerns the temperature of breakage of the disulfide bonds of the hair. In the case of a flat iron, a thermochromic functional indicator makes it possible to prevent, for example, any risk of burning with the hot base (in particular when the iron reaches its operating temperature and the indicator light of the thermostat turns off).
In this field, cooking containers are already known which comprise temperature indicators enabling the temperature of the internal surface of the containers to be known. These indicators generally include a temperature sensor integrated at the base of the cooking container and connected by conductors to an electrical unit comprising a screen for displaying the temperature measured by the sensor. Owing to such means, the user is constantly informed of the temperature of the cooking surface of the container, so that he or she is capable of detecting the precise moment at which the cooking surface has reached a sufficient temperature in order to successfully “sear” meat. In addition, the fact that the temperature of the cooking surface of the container is displayed also enables the user to be alerted that the article is hot and presents a burn risk to people.
Such temperature indicators nevertheless have the disadvantages of being complicated and presenting problems of electrical insulation, in particular with respect to washing water.
In some other known embodiments of heating articles, the temperature indicator consists of an organic thermochromic substance capable of suddenly changing colour at a certain temperature. It is in particular a micro-encapsulated thermochromic pigments old under the trade name ChromaZone®. This pigment changes from a coloured state to a colourless state when the temperature increases, due to a change in the physical state (change from the solid state to the liquid state of the solvent inside the microcapsule). However, this thermochromic pigment used as a temperature indicator has the disadvantage of not being placed directly on the heating surface of the article. Thus, for example, in the case of a cooking article, the thermochromic pigment is placed n the base of the handle, on an external edge of the article, or on the flat edge, so that the change in colour of the chemical compound is not a real indication of the temperature reached by the cooking surface. As this type of thermochromic compound is not resistant to temperatures above 200° C., it cannot be positioned directly on the cooking surface, of which the temperature reaches at least 200° C. on each use. If this were the case, such a compound would be quickly and irreversibly damaged.
The terms thermochromic substance, mixture or composition refer, in the sense of this invention, to a substance, mixture or composition that reversibly changes colour according to the temperature.
The French patent FR 1 388029 belonging to the applicant is also known, which describes a cooking utensil equipped with a heat indicator consisting of a thermosensitive body reversibly changing colour according to the temperature, in which said heat indicator is formulated in a non-stick coating, in particular consisting of poly-tetrafluoroethylene. A thermostable pigment can also be incorporated in the cooking utensil as an indicator so as to enable the relative change in colour of the thermosensitive indicator, and therefore the change in temperature, to be assessed.
The term thermostable pigment refers, in the sense of this invention, to a mineral or organic compound, which shows a very minor change in tint when it is subjected to a temperature increase within a given temperature range.
However, the simple association of a thermostable pigment and a thermosensitive pigment does not make it possible to clearly distinguish the change in temperature.
To overcome these problems, the applicant has developed a heating article comprising a surface covered by a base coating consisting of thermostable resin, which is resistant at least at 200° C. and on which a design based on a thermostable resin is applied, which is resistant at least at 200° C. This design includes at least two patterns, one containing a thermochromic chemical compound that darkens as the temperature increases, and the other containing a chemical compound that is also thermochromic, but that becomes lighter as the temperature increases. This article is the subject of the European patent EP 1121576.
The simultaneous use of these thermochromic chemical compounds in contiguous areas actually makes it possible to improve the visual perception of the change in temperature of the cooking surface of the heating article. However, this perception can be difficult: on the one hand because the two reds have chromatic values that are similar at room temperature, and on the other hand because the colour confusion occurs in an area with a heat amplitude of at least 50° C. Thus, the precision of the measurement and the ease of reading are not easy, in particular for people without special training. Therefore, the users have a tendency to overlook the information provided by this indicator.
The fact remains that good monitoring of the temperature during cooking of foods is necessary for sanitary and gustatory reasons, and to limit occasional overheating, which can weaken the coating of the cooking article.
In addition, semiconductors (SC) are also known, of which the properties make it possible to envisage a progressive change in colour as the temperature increases, according to the sequence white→yellow→orange→red→black.
The term thermochromic semiconductor (SC) in the sense of this invention refers to a semiconducting compound that shows a change in colour as the temperature increases. The progressive and fully reversible thermochromic character of these semiconducting compounds is associated with the decrease in the width of the forbidden band of the semiconductor due to the dilation of the material.
In the fields of application envisaged for this invention, a heating article of the cooking article or iron type is typically used in a range of temperatures between 100° C. and 300° C. However, in this temperature range, the changes in colour of the SCs remain limited and indistinct and are limited to the following changes: from pale yellow to bright yellow (Bi2O2), from yellow-orange to red-orange (V2O5), from red-orange to very dark red (Fe2O3), and so on. It can thus be seen that, even if it is possible to obtain the different colours of red, the perception of the thermochromic effect remains difficult and imprecise.
To overcome these problems of perception of the thermochromic effect of the semiconductors, in particular in temperature ranges (between 100° C. and 300° C.) that are typically the temperature ranges of use of heating articles such as cooking articles or irons, it is known to combine one or more thermochromic SCs with one or more thermostable SCs.
The main effect of this combination of thermochromic semiconductors and thermostable semiconductors is that the range of tints obtainable is thus particularly broad. But in addition, the perception of the change in tint is also increased: the mixture of a thermochromic semiconductor that will change from off-white to bright yellow can then, if it is associated with a blue pigment, change from cyan-blue to citron-green. As the maximum sensitivity of the human eye is centred on wavelengths corresponding to green, this mixture will not necessarily show more colorimetric parameter changes than the thermochromic pigment alone, but the human eye will perceive this change better.
The European patent EP 0 287 336 and the European patent EP 1 405 890 disclose the development of mixtures consisting of one or more thermochromic semiconductors and one or more stable pigments in order to obtain complex compositions with enhanced thermochromic properties. More specifically, in the patent EP 1 405 890, bismuth oxide Bi2O3 (thermochromic) is combined with CoAl2O4 (thermostable, blue), in a ratio of 15:1, with the pigments being bound by a potassium silicate. The coating containing this mixture is blue at room temperature and becomes orange at 400° C.
Admittedly, these mixtures of thermochromic semiconductors with thermostable pigments have the advantage of conferring on the coatings containing them a reversible thermochromism with improved visibility and precision.
However, certain thermochromic semiconducting pigments have the major disadvantage of not being compatible, during use on a heated surface, with oils or grease: they are said to be lipid-sensitive. Indeed, for example, the semiconducting metal oxides are easily reducible under heat in the presence of oil or grease and the compounds formed after this reduction reaction are no longer thermochromic.
It is therefore necessary to protect the thermochromic semiconductors that are lipid-sensitive under heat, so as to make them inert to an oil or a grease capable of reducing the thermochromic SCs.
It is known to a person skilled in the art, specialized in pigment-based compositions, to protect pigments by coating them with a mineral shell (or envelope). Such a technique is commonly used in numerous fields, such as, for example, cosmetics or paints. Thus, the French patent FR 2 858 768 describes a hair care formulation, comprising, as pigments, metallic particles coated with an oxide shell (silver particles coated with SiO2). These pigments make the hair shiny for a long period of time. In paints, the photocatalytic effect of the titanium oxide is decreased by coating by means of a silica mineral shell and the acid resistance of the ultramarine blue pigments is improved (artificial mineral pigments obtained by heating soda, clay and sulphur).
Finally, also in this field, it is possible to create pigments with an effect (metallic, pearled, and so on.) with an improved durability, by coating with a mineral shell based on silica, cerium oxide or iron, for example.
In addition, it is also known to a person skilled in the art that the protections provided by a silica-based coating to commercial yellow pigments based on mixed bismuth and vanadium oxide BiVO4 (in particular, the pigments sold under the trade names Sicopal Yellow K1160FG® of BASF, Lysopac Jaune 6613P® of CAPPELLE, and Vanadur Plus 9010® of HEUBACH, etc.) enable better colour stability under temperature and make it possible to use them in extruded plastics. The processes for producing these protected pigments are described in U.S. Pat. No. 4,063,956 of the Du Pont company, U.S. Pat. No. 4,851,049 of the BASF company, U.S. Pat. No. 5,851,587 of the Bayer AG company, and EP 1086994 of the Cappelle company. The coatings of the pigments, which re dense and continuous coatings containing in particular silica, are deposited by precipitation. Indeed, the yellow pigments inside the coating have a thermochromic character ranging from bright yellow at room temperature to red-orange at 300° C., but they cannot be used in a mixture combining a thermochromic pigment with a thermostable pigment because their colour is so bright and intense that it neutralizes the colour of any other added pigment.
Finally, the American U.S. Pat. No. 2,285,366 describes different synthetic pathways of an impenetrable silica shell around particles of all sorts of compositions. The coating can be done by silicic acid (“active silica”) condensation from soda silicate by precipitation, or from methyl-orthosilicate (or TMOS) by sol-gel, or from hydrolyzed SiCl4. These techniques of coating by precipitation or by sol-gel are still widely used because they enable shells (or envelopes) of a very wide variety to be created: metal oxides (of metals Al, Si, Ti, B, Mg, Sn, Mn, Hf, Th, Nb, Ta, Zn, Mo, Ba, Sr, Ni, Sb), and phosphate or pyrophosphate of Zn, Al, Mg, Ca, Bi, Fe, Cr and so on.
However, there is no mention in any of these documents of transparent protection, with specific characteristics to thermally resist temperatures that may be very high in the presence of oil or grease.